Abstract: Obesity, a major public health problem present in nearly one third of American adults, has been reported to increase risk of death in motor vehicle crashes (MVCs). Various mechanisms for the increased risk have been proposed, including: 1) momentum effects;2) obesity-related comorbidities such as cardiovascular disease (CVD), diabetes, and sleep apnea;3) post-treatment problems in the obese;and 4) vehicle design. Recent research we have conducted identified gender differences in the relationships between MVC fatality and body mass index (BMI): men (but not women) had an increased risk of death at both ends of the BMI continuum. We hypothesize that this differential association may be caused in part by differences in body weight and shape, center of gravity, and different accuracy of BMI as an obesity diagnosis standard for men and women. To evaluate this hypothesis, this project will use national MVC databases and geometric magnetic resonance imaging (MRI) assisted modeling of body variations in computer crash simulations. Specific Aim 1 is to test the effects of momentum, obesity-related comorbidities, and post-treatment on MVC outcomes for men and women in BMI and weight models using MVC data. We hypothesize that the gender differences of the influence of obesity on MVC outcomes will be mainly caused by differences in momentum effects and the co-morbidities related to obesity between men and women. Specific Aim 2 is to predict the effects of body shape and center of gravity on MVC injuries in men and women using computer crash modeling. We hypothesize that the gender differences in body weight and fat distribution, i.e., body shape, interacted with changes in velocity during the crash will have a great impact on injury severity and pattern between men and women. The findings of this project will advance our understanding of these two leading causes of death in the US. Moreover, it will provide groundwork for revisiting Federal Motor Vehicle Safety Standards (FMVSS) to protect as great a population as possible. Current FMVSS are based on standard dummy sizes (the 50th percentile male Hybrid III Crash Dummy, 1.78 m, 77 kg, BMI = 24.3 kg/m2). Given the high proportion of obese drivers and the variable effects of weight distribution, it is important to consider as well larger body size, greater BMI, and more shape variations in determining standards for traffic safety and public health.